There is a conventional method of pivoting an arm of an industrial robot by directly driving pivot axes of the arm with driving motors through speed reducers. However, this method is not suitable for those robot having a heavy arm or load because an unbalanced moment at the arm increases the load to the pivot axes. Therefore, in an industrial robot having relatively heavy arms and loads, parallel links are normally constituted by an arm A which corresponds to a driven bar, an arm B, driving bar C. and a connecting bar D as shown in FIG. 5.
However, when the arms are configurated using parallel links, there is interference between the arm B and connecting bar D and the operating range is thus narrowed. As a solution to this problem, the description of U.S. Pat. No. 4,329,111 and unexamined Japanese published patent application No. 59-110578 disclose a type wherein the swinging plane of driving bars and driven bars are shifted.
in the above-described conventional configuration, however, when an articulated arm is driven, a driving motor of a second parallel link constituting an arm at the end portion must be secured to the end of a first link supporting a second parallel link to constitute an arm. This increases the weight of the first link and, consequently, the load applied to the driving motor of the first link. Thus, a problem arises in that the rigidity of the first link must be high and the capacity of the driving motor must be large.
It is an object of the present invention to provide an arm-driving device wherein the arms are lightweight so that the operation of the arms can be quick, and wherein the interference of a parallel link driving a second arm is eliminated to enable expansion of the operating range.